Scroll down to 2-Factor Authentication and then click inside the box to enter your mobile phone number, then click Verify. You will then be sent an SMS message with a phone verification code.

You will be prompted to enter your phone verification code. After entering this, click Submit.

If the code matches, you will get a confirmation message that your 2-factor authentication is now active. You will also be given a one-time backup code that you can use just in case you don't have your phone or the authentication message does not reach you. Save this code and keep it in a secure place. Your 2-factor authentication has now been activated and will be valid for 30 days.